The mono- and diesters of fatty acids and polyhydroxylic alcohols, particularly glycerin, are compounds extensively utilized as emulsifying and surface-active agents in the food, cosmetic and pharmaceutical industries. These compounds can be obtained in two different ways: A) by direct esterification between the fatty acid and the polyhydroxylic alcohol; B) by transesterification between a polyhydroxylic alcohol and a fat (triglyceride) or a fatty acid methyl ester. From the industrial point of view, the transesterification processes are the most important for the preparation of mono- and diesters of fatty acids and polyols.
Transesterification is a reaction which takes place, as is indicated in the schematic, between an ester (1) and an alcohol (2) giving rise to a new ester (3) and another alcohol (4).

In general transesterifications can be catalyzed by both acids and bases. However, in many cases the use of basic catalysts is preferable, since acid catalysts can cause, depending on the structure of the alcohols which intervene in the process, secondary reactions such as isomerizations or dehydrations.
In transesterification catalyzed by bases the preferred catalysts are conventional bases such as KOH, NaOH, alkaline alkoxides such as NaOC2H5, NaO-t-C4H9, or NaHCO3, Na2CO3, and Ca(OH)2. This type of catalyst is difficult to eliminate from the end product and in addition they are not reusable.
C. M. Gooding et al., H. W. in the patent U.S. Pat. No. 2,197,339, describe a method for the transesterification of coconut oil with glycerin using NaHCO3 and Na2CO3 as catalysts. They obtain a mixture of esters and salts of fatty acids which are then made to react with different chlorohydrins like for example chlorohydrin of glycerol, giving rise to the formation of a monoester and NaCl.
Monta nola Martinez et al., describe in the Spanish patent application P-9001084 “Procedure for the preparation of fatty acid monoesters with glycols”, assigned to Kao Corporation, a method for the transesterification of esters of fatty acids with glycerin using dry Na2CO3 as catalyst.
Also in the European patent EP-0200982 (Henkel) the transesterification of triglycerides with alcohols is described at temperatures of between 60-75° C. and atmospheric pressure in the presence of Na2CO3 or NaHCO3.
In the patent EP-1260497 (2002), a process is described for obtaining monoesters of polyhydroxylic alcohols with high yields and purity. The transesterification between vegetable oils and polyols is performed in the presence of a solvent (tert-butanol or tert-amyl alcohol) at temperatures of between 160 and 200° C. and using base catalysts such as salts (oxides, hydroxides, carbonates, alkoxides and acetates) of alkaline and alkaline-earth metals and nitrogenated bases.
The transesterification processes presented above have serious drawbacks, like for example the use of solvents that have to be completely removed from the end product, or the use of homogeneous basic catalysts that must be neutralized and the resulting salts eliminated from the reaction medium, as well as the impossibility of reusing the catalyst.
The use of heterogeneous basic catalysts greatly simplifies this type of process. The heterogeneous basic catalyst is easily eliminated from the reaction medium by filtration, it allows the possibility of reuse and its application is also possible in fixed bed processes.
Corma et al., describe the use of heterogeneous basic catalysts of the type of alkaline-earth metal oxides, calcinated hydrotalcites, zeolites, sepiolites and zeotypes exchanged with alkaline cations as catalysts in the transesterification of mono—or polyhydroxylic alcohols with fats of animal or vegetable origin (Spanish patent application P9601087; Journal Catalysis 173, 315 (1998)). The transesterification of glycerin with methyl esters of fatty acids has also been performed using solid catalysts with a basic nature. Barrauti et al., describe the transesterification of methyl esters with glycerol in the presence of oxides such as MgO, CeO2 and La2O3, as well as MgO doped with alkaline metals (Li/MgO and Na/MgO) (Applied Catalysis A, 218, 1 (2001); Catalysis Today 75, 177 (2002)).
The present invention refers to a procedure for transesterification between polyhydroxylic alcohols and fatty esters, preferably triglycerides and methyl esters, using heterogeneous basic catalysts based on combinations of mono- and trivalent metal oxides and in combinations of di- and trivalent hydrated metal oxides.
With this type of mixed oxide higher reaction speeds and better selectivities to monoesters are achieved than with those known up to now, formed by a mixed oxide of a monovalent metal and a divalent metal which, in principle, would seem more suitable to carry out the transesterification according to its basicity.
It has been proven and described also that for a transesterification process, more active catalysts are obtained than the conventional mixed oxides formed by mixed oxides of divalent and trivalent metals, if these are rehydrated with a determined quantity of water.